[ServiceWorker] Add error handling code fetch-mixed-content-iframe.html to avoid timeouts.

BUG=426047

Review URL: https://codereview.chromium.org/672423003

git-svn-id: svn://svn.chromium.org/blink/trunk@184333 bbb929c8-8fbe-4397-9dbb-9b2b20218538